1. Field of the Invention
The present invention relates to an additive for fluxes and soldering pastes, a flux containing the additive and a soldering paste containing the flux.
In the field of electronics, fluxes are widely used for the purpose of removal of an oxide film on a metallic base. The flux of the present invention is a novel flux which never corrodes metallic bases and never cause sticking even though it remains on metallic bases after soldering.
2. Description of the Related Art
In a general method for soldering electronic parts on a metallic base (for example, a copper base) for electronic circuits, a surface of the base is treated with a flux to remove oxide film and then the surface is dipped into a melted solder bath.
However, this method requires two steps: (1) treatment with the flux and (2) dipping into the solder bath. Further, in this method, some of the electronic parts put on the base occasionally become dislodged from the base in the course of the steps.
In order to solve these problems, the method using a soldering paste containing a flux and solder particles has been developed. According to the method, the soldering paste is applied on a base by, for example, a dispenser and then heated or irradiated with ultraviolet light to complete soldering. Since the soldering paste is viscous, electronic parts are easily held on the base.
However, even the above method has some disadvantages. The soldering paste contains an activator such as organic acids and organic acid salts in order to completely remove oxide film on the base. After soldering, the soldered base has to be washed to remove the activator because the corrosion of the base with the remaining activator continues after completion of the soldering operation.
In general, a solvent of fluorohydrocarbons such as Freon and water are used for this washing. However, the use of fluorohydrocarbons tends to be prohibited because they have harmful effects on the atmospheric environment. Further, if water is used for the washing, the resulting waste water is polluted with heavy metals and cannot be discharged without removal of the metals.
European Patent Publication No. 0413312A1 discloses a non-washing type flux and a soldering paste containing this flux which does not corrode metallic bases even though the soldered bases are not washed after soldering.
A heat treatment for soldering electronic parts on bases for electric circuit is conducted at 200.degree.-250.degree. C. for 2-5 seconds in order to prevent heat damage to the electronic parts.
Since the heat treatment period is relatively short, when the amount of the flux described in the above EP Publication is small, a part of the activator does not react with the flux and remains on the bases. If the amount of the flux is increased to promote the reaction of the activator with the flux, the excess and unreacted flux remains on the bases and causes sticking.
Therefore, an object of the present invention is to provide a soldering paste which contains an activator and does not continue to corrode a base after soldering even though the soldered base is not subjected to a washing step.
Another object of the present invention is to provide a flux which contains an activator and does not continue to corrode a base after soldering even though the flux is contained in a soldering paste and the base soldered with this paste is not subjected to a washing step.
A further object of the present invention is to provide an additive for a flux and a soldering paste containing an activator, which is contained in the flux and the soldering paste and deactivates the activator completely in the course of soldering of electronic parts in a relatively smaller amount.